Thermoplastic Piping Stress Analysis
HDPE, PP, PVC, PVDF, PE-RT, and Other Thermoplastics

Overview

This function applies to non-reinforced rigid thermoplastic piping systems.

For reinforced thermoplastic piping, use the fiberglass piping code.

For flexible plastic piping, use the flexible plastic piping function in START-Elements.

Key differences between thermoplastic and steel piping analysis:

• Allowable stress depends on service life and temperature, calculated using the equation: . Material-specific A, B, G, J factors are stored in the material database. See Stresses in Piping for calculation details.

• Thermal expansion coefficients are significantly higher than steel, resulting in greater thermal expansion (ε₁)

• Pressure-induced expansion (ε₂) from the Bourdon effect is substantial and must be considered

• Chemical swelling strain (ε₃) may require consideration. Specify swelling values in pipe additional properties

• Stresses are calculated using creep modulus (Ecm) that varies with service life and stress level. Support loads use 100-minute modulus (Esm). Occasional loads use 0.1-hour modulus (Est)

• Operating condition analysis uses average creep modulus between installation and operating temperatures

• Poisson's ratio is temperature-dependent, approaching 0.5 at elevated temperatures

• Allowable stress incorporates chemical resistance, installation condition, safety, and joint strength factors

• Wall thickness checks are performed only for straight pipes, not fittings

PASS/START-PROF uses the DVS 2205/2210 method for thermoplastic piping analysis.

Axial Expansion

Total axial expansion in thermoplastic pipes:

Thermal expansion (ε₁):

k - Temperature range factor from pipe additional properties, α - Thermal expansion coefficient from material database

Pressure expansion (ε₂):

t - Wall thickness, D - Outer diameter, μ - Poisson's ratio, p - Operating pressure, Ecm - Average creep modulus

Swelling expansion (ε₃):

Creep Modulus

Creep modulus depends on service life (Project Settings), temperature, and stress level.

Three creep modulus types are used:

• Average creep modulus (Ecm) - For sustained, operating, and cold condition stress calculations

EminT - Creep modulus at ambient temperature, service life, and current stress from material database

EmaxT - Creep modulus at operating temperature, service life, and current stress from material database

• Average creep modulus (Esm) - For support and equipment load calculations at operating temperature

E1.6minT - Creep modulus at ambient temperature, 100-minute service life, and current stress

E1.6maxT - Creep modulus at operating temperature, 100-minute service life, and current stress

• Average creep modulus (Est) - For occasional load calculations (wind, seismic, water hammer, etc.)

E0.1minT - Creep modulus at ambient temperature, 0.1-hour service life, and current stress

E0.1maxT - Creep modulus at operating temperature, 0.1-hour service life, and current stress

Creep modulus relationships in material database:

Allowable Stress Calculation

Sustained Load Stresses (Weight, Pressure)

Ky - Safety factor from pipe properties

Kc - Joint strength factor from pipe properties

Kx - Chemical resistance factor from pipe properties

Kp - Installation condition factor from pipe properties

Kt = 1, Kcyc = 1

- Nominal long-term allowable stress

A1, B1, G1, J1 - Characteristic factors for left curve from material database

A2, B2, G2, J2 - Characteristic factors for right curve (set to zero if single curve used)

- Service life from project settings

- Operating temperature from pipe properties

- Temperature safety factor from material database (K20 for ≤20°C, Kope for >20°C)

Operating Condition Stresses (Primary + Secondary Loads)

- Service life from project settings

Kt - Secondary stress factor from material database (PVC/PVC-C: 1.75, PE/PE-RT: 2.5, PVDF: 3.5, PP: 2.5)

Kcyc - Fatigue factor

N - Total cycles (annual cycles × service life) from Temperature Cycles

Kcyc range: 0.4 to 1.0

Other factors follow sustained load calculation rules

Cold Condition Stresses (Primary + Secondary Loads)

- Service life from project settings

- Ambient temperature from Project Settings

Other factors follow operating condition calculation rules

Test Condition Stresses

Ky = 1, Kx = 1, Kp = 1, Kt = 1, Kcyc = 1

= 24 hours

- Test temperature from pipe properties

- Occasional load safety factor (Kacc) from material database

Occasional Load Stresses

Ky = 1, Kx = 1, Kp = 1, Kt = 1, Kcyc = 1

= 24 hours

- Operating temperature from project settings

- Occasional load safety factor (Kacc) from material database

Stress Equations

Hoop Stress

Axial Stress

Torsion Stress

Equivalent Stress

p - Pressure

D - Outer diameter

t - Wall thickness

A - Cross-sectional area

Z - Section modulus

Mt - Torsion moment

Mi - In-plane moment

Mo - Out-of-plane moment

- Ring bending stresses from nonlinear FEM model

Stress Intensification Factors

Bends:

Straight Tees:

Reducing Tees:

Reducers:

Thermoplastic Pipe Failure Example

PVC-C pipe crack caused by excessive torsion from thermal expansion. START-PROF analysis during design can prevent such failures.

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